How to include the name of forum thread in the forum notification email? By default the form notification email shows "New post is submitted" and when getting multiple emails for submitted posts it is not clear which email related to different forum threads.

To insert the Thread name, url or other information it is needed to intercept the email before it is sent and modify the subject. Making modifications to the Subject of the email from the editable templates in Sitefinity backend will not give a result because the subject is plain text and will not evaluate any macros that will bring up the thread theme unless it is written manually in the subject.

To apply this customization you need to do this trough code:
1. Refer to How to Extend Sitefinity SmtpSender KB article:
Add the SmpSenderCustom in your project and register it for use in Sitefinity as explained in the KB article.

2. In SmpSenderCustom SendMessage method filter add filtering that will capture only the emails with subject "New post is submitted" this way the logic will intercept only emails that come from forums notifications.
I have prepared a sample below:

public override SendResult SendMessage(IMessageInfo messageInfo, ISubscriberRequest subscriber)
{
if (messageInfo.Subject == "New post is submitted")
{
//need to get the Thead name from the BodyHtml e.g. <title>New post in Thread1</title>
var getHtml = messageInfo.BodyHtml;
var checkString = "New post in";
var threadTitleIndex = getHtml.ToString().IndexOf(checkString);
var threatTitleNonCleared = getHtml.ToString().Substring(threadTitleIndex).Substring(checkString.Length, 100);

var indexOfTitleTag = threatTitleNonCleared.IndexOf("</title>");
var threadName = threatTitleNonCleared.Substring(0,indexOfTitleTag);

messageInfo.Subject += " " + threadName;
return base.SendMessage(messageInfo, subscriber);
}

return base.SendMessage(messageInfo, subscriber);
}

The method modifies the subject of emails that contain the subject "New post is submitted" when user posts new post in thread. The code reads the name of the forum thread from the body of the email and appends it to the Subject of the email. This is done because in the body of the email you can specify {|Thread.Title|} and dynamically resolve thread name, which can`t be done in the Subject of the mail.
I am using the built-in email template for forum posts where the Thread name is defined in the template with this string: New post in {|Thread.Title|}, I have attached a screenshot of this, refer to the attached files.
If you have modified the template for forum posts modify the logic in SendMessage method to properly find the Thread.Title in the home. By default it is available in title tag like: <title>New post in Thread1</title> .